Cracking of neohexene to isoprene



Dec. 24, 1963 J. w. COLTON ETAL 3,115,531

CRACKING OF NEOHEXENE TO ISOPRENE Filed Oct. 2, 195a HYDROCARBONS TO USEAS FUEL IN m; PROCESS- r- GASOLINE FRACTION ISOPRENE -2 ,/a r2/ ,20 I 3I /e SEPARATOR osuvonm SEPARATOR. omcxsn. SEPARATOR SENATOR 112NEOHEXANE' TO DEHYDROGENATOR. I5 32 a HYDROCARBON SEPARATOR ISOMERJZERPARAFFINS 30 L? 52 rezonzxm: TO [44 DEHYDROGENATOR. I5

47 HYDRO GENATOR. 54 [4/ 46 49: 8 1 SEPARATOR I [so 1:..E PROPYLENESEPAR- 40 DIMERIZER OR F IZER Kw INVENTORS 42 4 57 JOHN WHITE COLTON- 1REXEL/DOV GERSON S. SCH/1F United States Patent 3,115,531 CRACKING 0FNEOHEXENE T0 ISOPRENE John White Colton, Pelham Manor, Rex E. Lidov,Great Neck, and Gerson S. Schaifel, Old Westbury, N.Y.,

assignors to Halcon International, Inc., a corporation of Delaware Filed(let. 2, 1958, Ser. No. 764,876 1 Claim. ((31. 260-680) This inventionis concerned with the production of isoprene. More particularly, thisinvention is concerned with the production of isoprene from neohexaneand its derivatives. More specifically, this invention is concerned withprocesses for the production of isoprene from neohexane comprising thesteps of dehydrogenating neohexane to produce neohexene anddemethylating said neohexene to yield isoprene.

It is known in the art that isoprene may be prepared from aliphatichydrocarbons in a variety of methods embracing several combinations ofsteps. Many of these methods require large quantities of unavailablematerials and produce relatively low overall yields, thereby renderingsuch processes uneconomical.

Another difficulty encountered in the art is the large heat energyrequirement in processes for the production of isoprene, particularlywhere the steps of dehydrogenation, cracking and distillation arenecessary to obtain the desired product.

It is a feature of the present invention to produce isoprene frommaterials available in abundant supply, such as from C hydrocarbonfractions which may be readily separated to yield precursors ofisoprene.

It is another feature of the present invention to utilize paraffinstreams, such as that produced by isomerization of light naphtha, as asource of neohexane. Mixtures of isomeric parafiins, from whichneohexane may be obtained by distillation, are frequently produced forthe manufacture of high octane gasoline, and are available at low costand in abundant supply, so that gasoline may be utilized as a startingmaterial for the production of isoprene, thereby providing an economicalmethod for producing isoprene.

It is another feature of the invention to utilize the hydrocarbons whichare formed as by-product in the production of isoprene, as a source ofheat energy for the process, thereby providing an economical method forobtaining fuel and minimizing power and utility costs for suchprocesses.

It will be realized that another feature of this invention is to providea process for the production of isoprene which using neohexane as theprincipal feedstock, makes possible ready recovery and recycle of thoseportions of the feedstock which do not undergo conversion in the firststages of the process.

It is still another feature of the present invention to produce isoprenein good yields from neohexane, taking advantage of the unique structureof neohexane, which forms only one olefin, neohexene, upondehydrogenation.

It is another feature of this invention to provide a novel method bywhich neohexane may be dehydro genated to produce neohexene which, uponcracking, yields isoprene.

It is a further feature of this invention to provide a novel method bywhich neohexene may be thermally cracked to produce isoprene.

A preferred embodiment of the invention comprises, in a process forproducing isoprene, the steps of dehydrogenating neohexane to form amixture of neohexane and neohexene, separating said neohexane from saidneohexene, recycling said neohexane to the dehydrogenating step,cracking said neohexene to produce a 3,115,531 Patented Dec. 24-, 1963mixture of isoprene and other hydrocarbons, separating isoprene and saidother hydrocarbons and using said hydrocarbons as a source of heatenergy for the process.

Still another preferred embodiment comprises the steps of separatingneohexane and other C hydrocarbons from a C hydrocarbon fraction,isomerizing the said separated C hydrocarbons, recycling the isomerizateto the separation step, dehydrogenating said separated neohexane to forma mixture of neohexane and neohexene, separating neohexane from saidneohexene, recycling said separated neohexane to the dehydrogenatingstep, cracking said neohexene to produce a mixture of isoprene and otherhydrocarbons, separating isoprene and said other hydrocarbons and usingsaid hydrocarbons as a source of heat energy for the process.

Another preferred embodiment comprises the steps of producing neohexanefrom precursors of neohexane, separating said neohexane from saidprecursors, recycling said separated precursors to the neohexanepreparation step, dehydrogenating said neohexane to form a mixture ofneohexane and neohexene, separating neohexane from said neohexene andcracking said neohexene to produce a mixture of isoprene and otherhydrocarbons and recovering isoprene.

Other objectives of this invention will become apparent from thedescription thereof which follows:

FIGURE 1 is a schematic illustration of an embodi ment of the invention,in which isoprene is produced from gasoline.

FIGURE 2 is a schematic illustration of an embodiment of the invention,in which neohexane is produced from a C hydrocarbon stream.

FIGURE 3 is a schematic illustration of an embodiment of the invention,in which neohexane is produced from a precursor, propylene, bydimerization and hydrogenation.

Referring to FIGURE 1, gasoline is fed through line 10 into separator 12and neohexane is removed as distillate through line 13. The residue iswithdrawn through line 11. The distillate from line 13 is fed intodehydrogenator 15 and the products of the dehydrogenation are ledthrough line 16 into separator 18. Neohexene is removed from theseparator and fed through line 19 into cracker 2h. The unreactedneohexane is removed overhead from separator 18 and recycled throughline 21 to dehydrogenator 15. The neohexene is cracked in cracker 20 toform isoprene, and other hydrocarbons, such as methane and ethane. Thismixture is fed through line 22 into separator 23 where isoprene productis removed through line 27, the uncracked neohexene product is recycledthrough line 25 to cracker 2t) and the other hydrocarbons are removedthrough line 24 to be used as fuel in the process; for example, cracker20, dehydrogenator 15, or wherever else fuel is required in the process.

Referring to FIGURE 2, a C paraffin stream is fed through line 39 intoseparator 31. Neohexane is removed through line 35 and other Chydrocarbons which are separated from the stream are led through line 32into isomerization unit 33. The C hydrocarbons are isomerized and theisomerizate is recycled to separator 31 through line 34. The neohexane,which is contained in the isomerizate is removed through line 35 todehydrogenator 15 and the dehydrogenation and cracking steps are carriedout as described in FIGURE 1.

Referring to FIGURE 3, propylene is fed through line 40 into dimerizer41. After dimerization the products are led through line 42 intoseparator 46. Unreacted propylene is recycled to the dimerizer throughline 44. The heavier products are removed through line 45. The C olefinscontaining neohexene are fed through line 47 to hydrogenator 48 wherethey are hydrogenated and passed through line 49 into isomerizer 50. Theisomerization products are fed through line 57 into separator 52 and theneohexane is wremoved through line 54 to dehydrogenator and thedehydrogenation and cracking steps are carried out as described inFIGURE 1. The remaining C paraffins are recycled through line 53 to theisomerizer 50.

Neohexane may also be produced by isome'rizing C parafiinic hydrocarbonsover an aluminum chloride catalyst in the presence of HCl. Theisomerization is usually carried out at a temperature of about 100 C. inthe presence of hydrogen at a pressure of 1000 p.s.i.g. The hydrogen isemployed to inhibit disproportionation and cracking. Naphthenes orbenzene may be used instead of hydrogen to inhibit cracking. Othercatalysts, such as metallic oxides, for example, nickel or cobalt onsilica alumina, may be utilized for isomerization at pressures of aboutatmospheres in the presence of hydrogen. Such reactions may be carriedout at temperatures in the range of about 300-450" C.

Another method of producing neohexene which may be employed is theisomerization of dimethyl butenes to neohexene. Neohexane may also beproduced by reacting iso-butane and ethylene.

In still another method, neohexene is produced by reacting tertiarybutyl chloride with ethylene over an aluminum chloride or ferricchloride catalyst, hydrolyzing the resulting product to thecorresponding tertiary hexanol and pyrolyzing the hexanol to produceneohexene.

In another embodiment of the invention, neohexene is produced directlyfrom methyl ter-butyl carbinol by dehydration and then cracked toisoprene.

Other methods of producing isoprene from neohexane precursors utilizingthe teaching of the present invention will become apparent as a resultof these disclosures and it is intended that the invention embrace theformation of neohexane from other such precursors, that is, substanceswhich precede formation of neohexane.

The invention is illustrated by but not restricted to the followingembodiments.

As used herein parts and percentages are by weight, unless otherwisespecified.

Example I A neohexane sample is passed over a platinum alumina catalystat a temperature of about 600 C. at a contact time of about 2.8 seconds.

The product condensate is separated into neohexene, which boils at 41.2"C. and all other constituents, including mostly unreacted neohexane butsome other olefins and paraffin isomers. These may be saturated withhydrogen in a separate catalytic step and recycled to the isomerizer.

The resulting neohexene is diluted with nitrogen at 'a nitrogen toneohexene mol ratio of 7.611 and is fed into a reactor tube having aratio of length to diameter of about 50:1 which is packed with siliconcarbide granules for approximately one-half of its length at thedischarge end, at a rate of 10.7 volumes of liquid per hour per volumeof reactor. The cracking is carried out at a temperature of about 800 C.and there is obtained a conversion of 77.5% and a selectivity toisoprene of 4 Example 2 Example 1 is repeated using a chrome aluminacatalyst instead of a platinum alumina catalyst. Substantially similarresults are obtained.

The dehydrogenation step is carried out in an acidfree catalyst systemat a temperature of broadly 450 to 900 C., preferably 500 to 650 C. anddesirably 575 to 625 C. The pressure is broadly 0.1 to 10 atmospheres,preferably 0.5 to 1.5 atmospheres and desirably close to atmospheric.The contact time is broadly 0.2 to seconds, preferably 0.5 to 20seconds, and desirably 0.8 to 10 seconds. Temperature and contact timeare interdependent with short contact times preferred for hightemperatures and vice versa. The catalyst is platinum alumina, chromiaalumina or any other suitable acid-free dehydrogenation catalyst withwhich undesirable structural isomerization is prevented.

The cracking step is carried out at a temperature of broadly 650 to 900C., preferably 700 to 850 C. and desirably 760 to 800 C. The contacttime for cracking may be broadly 0.01 to 1.0 second. Packing such assilicon carbide granules or any other equivalent material may beemployed. The diluent may be steam or nitrogen or other inert gas. Steamis preferred in commercial operations, although nitrogen is used forconvenience in many laboratory experiments. The pressure is broadly 0.1to 10 atmospheres, preferably 0.5 to 1.5 and desirably close toatmospheric.

In view of the foregoing disclosures, variations and modificationsthereof will be apparent to anyone skilled in the art, and it isintended to include within the invention all such variations andmodifications except as do not come within the scope of the appendedclaim.

What is claimed is:

In a process for the production of isoprene, the step of cracking afeedstock concentrated in neohexene at a temperature in the range offrom about 650 to 900 C. and a pressure in the range of about 0.1 to 10atmospheres, at a contact time of about 0.01 to 1.0 second to produce amixture of isoprene and other hydrocarbons and recovering said isoprene.

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